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Beginning Assembly, Part 2 The Assembling!. Poorly Presented by Gleep. Dicks. How does Dicks work? Zenity dependancy Pushes text params onto the stack Stores mem addresses of params in registers

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dicks
Dicks
  • How does Dicks work?
  • Zenitydependancy
  • Pushes text params onto the stack
  • Stores mem addresses of params in registers
  • Calls execve to start Zenity and passes regs to Zenity with params for pop-up text box
binary coded decimal
Binary Coded Decimal
  • Represents a decimal number by coding each digit in binary
  • Unpacked BCD is one byte per digit
  • 00000011 00000100 00001001
  • 3 4 9
  • Packed BCD is one byte per two digits
  • 00100100 10010001
  • 2 4 9 1
floating point maths
Floating Point Maths
  • Floating point ops use special registers ST(0) thru ST(7)… 80 bit registers for data
  • 1 - 16 bit control register
  • 1 - 16 bit status register
  • 1 - 16 bit tag register to describe contents of data regs
  • 1 - 48 bit FIP register for next floating point op
  • 1 - 48 bit FDP FPU data pointer
  • 1 - 11 bit Opcode register
floating point reg ops
Floating Point Reg Ops
  • The floating point registers act as a stack. ST(0) is top of stack.
  • Fldsrc – loads 80 bit src onto FP stack
  • Fstdst – moves top of FP stack into dst
  • Info all will show all FP and SSE registers
floating point maths1
Floating Point Maths

((55.24 / 31) + (83.11 * 2.4)) / ((14.35 * 9) – (251.5 / 77.62)) = OMGLOLWUT?!?!

single instruction multiple data
Single Instruction Multiple Data
  • Aka – MMX, SSE, 3DNow!
  • MMX aliases the 8 FPU data regs as MM0-7 for 64 bit packed integers
  • SSE includes 8 new 128 bit regs XMM0-7 for 128 bit packed integers and floating point data
  • Useful for processing large amounts of data with one instruction
simd continued
SIMD Continued
  • MMX regs can store 8x8 bit byte ints, 4x16 bit word ints, or 2x32 bit double word ints
  • Movqsrc, dst – where dst is %mm0-7
  • SSE regs can store 16x8 bit byte packed ints, 8x16 bit word packed ints, 4x32 bit double word packed ints, or 2x64 bit quad word ints
  • Movdqusrc, dst – moves unaligned data into %xmm0-7
  • Movdqasrc, dst – moves aligned data. Using this with unaligned data will throw an error.
simd continued1
SIMD Continued
  • This was supposed to be a meaningful slide but then I got bored with SIMD.
  • Research it yourself if you need to code it in natively.
c structures in asm
C structures in ASM
  • For Loops
  • If/Then
  • While
  • Switch
using c libraries in asm
Using C Libraries in Asm
  • When using C functions in asm, need to link in C libraries containing the functions
  • Ld –dynamic-linker /lib/ld-linux.so.2 –o -lx - where x is /lib/libx.so
  • /lib/ld-linux.so.2 is a dynamic loader
  • -lc option would link to /lib/libc.so
compiling asm with gcc
Compiling Asm with GCC
  • Gcc –o
  • Automagically links everything
  • Must change _start label to main
unconditional branching
Unconditional Branching
  • JMP
    • 3 types… Short, Near, Far
    • Equivalent of Goto:
    • Short JMP is less that 128 bytes
    • Far JMP is to another code segment
    • Near JMP is everything else
    • Takes one memory label as a parameter
  • Call
    • Equivalent to function call in C
    • Call pushes EIP to stack
    • Call returns to main with the RET command
unconditional branching cont d
Unconditional Branching Cont’d
  • Interrupt
    • Hardware – used for I/O functions
    • Software
      • Used for accessing kernel functions
      • In Linux, uses Interrupt 0x80
      • In Windows, uses Interrupt 0x21
      • Parameters in registers determine which function is being called with what parameters
      • Ex: MOVL $1, %EAX
        • MOVL $0, %EBX
        • INT $0x80
conditional jmps
Conditional JMPs
  • JZ
  • JNZ
  • JGE
  • JLE
  • There’s a bunch of em… use the Googles
function calls
Function Calls
  • .type , @function
  • :
  • Ret
  • Call
  • Functions can be defined anywhere within asm file
  • Return values can be returned in regs or global vars
  • Can put functions in separate file, just add .globl statement after .type statement, and add function object file to linker statement
function prologue and epilogue
Function Prologue and Epilogue

Prologue

    • Function:
      • Pushl %ebp
      • Movl %esp, %ebp
      • Subl $8, %esp
  • Epilogue
      • Movl %ebp, %esp
      • Popl %ebp
      • Ret
enter and leave
Enter and Leave
  • Enter #bytes – used to perform function call prologue
  • Leave – used to perform function call epilogue
slide19
EBP
  • Main Process executes

ESP

  • Main Process calls New Function
  • Call places RET addr on stack
  • Prologue pushes EBP to stack
  • Prologue sets EBP to ESP
  • Prologue decrements ESP to make room for variables
  • New Function loads variables
  • New Function executes
  • Epilogue sets ESP to EBP
  • Epilogue pops old EBP from stack
  • RET pops old RET address to EIP
  • Main Process resumes
system calls
System Calls
  • Can be found in /usr/include/asm/unistd.h
  • Look up the needed input and return values in man 2
  • Can use strace to see background syscalls when running a program
  • Strace –p will attach strace to a running process
  • Use man 3 to see C function calls
inline assembly
Inline Assembly
  • You can embed assembly within C programs
  • Asm ( “code goes here”);
  • Must enclose the code in quotes
  • Can use the “volatile” keyword to tell compiler not to optimize it
  • Must use \n newline char if embedding more than one command
  • If coding ANSI C, use __asm__ instead
  • Asm ( “movl $1, %eax\n\t”

“movl $0, %ebx\n\t”

“int $0x80”);

calling libraries
Calling Libraries
  • If using asm libraries in C code, add the asm files onto compile statement
  • Gcc –o testprogtestprog.c func1.s func2.s
  • Can also assemble code into object file then add the object file to gcc
making libraries
Making Libraries
  • Can create a static library using the Ar command to add object files into an archive
  • Static library name syntax Libx.a
  • Ar r libmyfunc.a func1.o func2.o func3.o
  • Can see what is contained in a library using nm command
  • Nm –s libmyfunc.a
  • Compile by including the libx.a file on gcc command
making shared libraries
Making shared libraries
  • Shared library name syntax libx.so.
  • Create shared library with –shared option on gcc
  • Gcc –shared –o libmyfunc.so.1 func1.o func2.o func3.o
  • To compile using shared library, in same dir as program file use –L. option
  • Gcc –o testfunc –L. –lmyfunctestfunc.c
using shared libraries
Using shared libraries
  • The dynamic loader must know where to find any libraries needed for program function
  • Change LD_LIBRARY_PATH environment variable
  • Export LD_LIBRARY_PATH= “$LD_LIBRARY_PATH:.”
  • Or change the /etc/ld.so.conf file
  • After adding path to ld.so.conf, must run ldconfig as root
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